This invention relates generally to fiber optic couplers, and is concerned in particular with the fabrication of asymmetric fiber optic couplers
The simple symmetric fiber optic coupler, in which input light at any branch is split substantially equally between output branches, has little practical application to linear fiber optic buses and networks since only a trivial number of taps are possible before optical signal strengths become impracticably small. Larger linear fiber optic networks require asymmetric couplers in which tap-off coupling is substantially less than half, but tap-on coupling is much greater than the tap-off coupling. In the context of this specification, the terms "asymmetric coupler" and "asymmetric coupling" refer to coupling asymmetry of this kind. Such a requirement arises, for example, in networks where each component tied to a bus includes both an optical transmitter and an optical receiver, the receiver tapping the bus in front of the tap for the transmitter.
It has been proposed that asymmetric couplers having, e.g. a throughput power coupling factor, e.g. on a bus, of 95 and a tap-off coupling factor of 1.5%, might be fabricated by employing conventional techniques to merge two optical fibers of substantially different diameters or by simply reducing the nominal coupling factor of two similar fibers. The resultant couplers are satisfactory for single-mode applications, e.g. in basic telecommunication systems, but unacceptable for multi-mode operation because of significant power losses and the restricted proportion of modes which successfully couple to and from the tap fibers